Corona is a discharge caused by electrical overstress in insulating materials. The occurrence of corona is often associated with the electrical failure of the insulator or insulating materials. In solids the occurrence of corona can result in the deterioration of the insulating material.
In high voltage transmission lines, the insulating material in which coronal discharge occurs is the air surrounding the conductor or insulators of the transmission line hardware. Thus, in the context of transmission lines corona has been described by the American Standards Association as: a luminous discharge due to ionization of the air surrounding a conductor around which exists a voltage gradient exceeding a certain critical value. These discharges occur as the electrons from the intense electric field surrounding the conductor exceed a critical value. Normally, the electrons collide with air molecules such as oxygen or nitrogen and the electron bounces off with no transfer of energy to it. However, when the intensity of the electron exceeds a critical value the velocity of the electron is such that its collision with the air molecules is inelastic. Thus, an electron is knocked from an outer shell of the air molecules ionizing these molecules. The molecule with the missing ion is now a positive ion. The positive ions, e.g. nitrogen ions, are attracted towards the negative conductor in a sluggish manner due to their large size. These ions manage to capture free electrons which results in a quanta of energy being released as the energy level of a neutral molecule is less than that of the positive ion. The energy is radiated as an electromagnetic wave in the visible light range from the recombining of nitrogen ions with free electrons.
The coronal discharges result in deterioration of synthetic materials comprising insulators on transmission lines. The discharges also result in the loss of efficiency in transmitting current along such hardware and radio disturbances or interference caused such discharges. Increased expenses in the form of replacement insulators and related labor are also a result of coronal discharge.
In the past, numerous shielding and ring devices have been disclosed which spread and deconcentrate the electric field surrounding conductors and insulators to prevent coronal discharge. The majority of the control devices use some type of transverse mounting means such as a suspension clamp or a yoke plate connected to a suspension clamp to hold the control devices in place. These mounting means are complex to manipulate and utilize numerous pieces such as fasteners and u-shaped clamps. The manipulation of complicated mounting means makes installing and removing the control devices difficult for the line worker. Line workers are in a precarious work environment high in the air where the manipulation of mounting means increases the length of time it takes to complete a job. Should one of the pieces become completely disengaged and fall to the ground, the line worker must have a replacement piece handy or retrieve the missing piece to complete the job. Occasions may arise wherein the control devices must be installed or removed in a hot line situation and the added manipulation of the mounting means increases the risk of injury to the line worker. There is a need in the transmission line hardware for a corona control device or ring which has a mounting apparatus for simple installation and removal of the device.
Placement of control devices relative to the end fittings and weather sheds of the polymer insulator end fitting is extremely critical for the device to perform properly. Field calculations and laboratory tests are conducted to determine the precise location of the control devices for particular transmission hardware at a rated voltage. However, it is the line worker and not the engineer who installs the control device on the insulator end fitting. Though competent and fully trained, it is possible for a line worker to install many of the existing control devices in an inverted manner which results in improper positioning of the ring or shield. Improper installation can lead to ineffective control as position of the ring is critical and degradation of insulator material may result. There is a need for in the transmission line industry for a control device such as a shield or ring which prevents inverted installation by a line worker.